The present invention relates to the reception of video signals, and more particularly, to the storage of teletext type digital data.
Teletext is an ancillary service of television stations designed for the transmission of written information in addition to video information. This written information is displayed on the screen of a television set in the form of teletext pages. Each page is conveyed in the form of a variable number of data packets transmitted in the time intervals corresponding to the frame flyback between two images. A teletext service including a set of numbered pages is thus transmitted non-stop and in cycles. The transmission can be made in the chronological order of pages, but this is not necessary.
A full teletext service of a television station may comprise, for example, 500 pages. Page 100 of the service generally corresponds to a contents page of the service indicating the theme and the reference number (or page number) of certain pages of the service. The total pages of the service is broadcast in cycles, and a broadcasting cycle approximately lasts several tens of seconds. This enables a user to watch the service without necessarily storing all the pages. Broadcasting in cycles also enables the broadcasting station to regularly update its teletext service.
In general, to request the display of a page of the service, the user indicates the page number via a remote control. Through shift commands of the xe2x80x9cnext page/previous pagexe2x80x9d type, the user can furthermore move from one page to another in the service. With known navigation systems, such as the TOP or FLOF systems, the user can move about more easily in the service and request the display of a page of his choice without necessarily knowing the corresponding reference number.
For example, with the TOP system, the service is divided into blocks (or subject headings, such as sports) and each block is divided into one or more groups (or sub-headings, such as tennis) comprising a variable number of pages. The system works as follows. When a page of the service is displayed, the TOP navigation system displays a complement at the bottom of the screen. This complement comprises a list of pages close to the page displayed in the form of color links, with each link referring to a nearby page. The user can then very easily access the pages of a group or a block by a remote control. The user no longer needs to search for their reference number by going through the contents page, for example.
Here and throughout the rest of the document, the expressions xe2x80x9cclosexe2x80x9d, xe2x80x9cnearxe2x80x9d, xe2x80x9cnearbyxe2x80x9d and xe2x80x9cneighboringxe2x80x9d must be understood in the broad sense. A page Y is said to be close to a page X if, when the page X is displayed on the screen, there is a high probability that the user will request the display of the page Y.
For example, pages having consecutive reference numbers or almost consecutive reference numbers are close to one another. For example, pages 198, 199, 201, 202 are close to page 200. Similarly, with the TOP navigation system, the first pages of the block, the group, etc., to which the page X belongs are also pages close to the page Y.
A known circuit 10 for the reception and display of a teletext service comprises, according to FIG. 1, a decoder 11, a filter 12, a memory 13, a data memory 14, a display memory 15, display means 16 and a screen 17. The circuit 10 also comprises a data detection circuit coupled to an antenna (not shown) to receive a video signal, extract digital data on a teletext service from this video signal and supply the digital data to the decoder 11.
The memory 13 comprises a list of requested pages identified by their respective reference numbers. The list of requested pages contains references to pages close to the page being displayed, namely pages for which the probability is high that the user will request the display rapidly. The reference numbers contained in the list may be constant or variable. The contents of the list are updated as a function of the page being displayed and the navigation system used, and the updating is done whenever the display of a new page is requested by the user or whenever an updating of the information on the navigation system is received. The updating can be received after the display of a new page. The updating of the list of pages requested is done by an external list management circuit, which is not shown in FIG. 1.
The decoder 11 receives the digital data pertaining to a page of the service, decodes them and extracts the corresponding reference number. The filter 12 compares the reference number of the page received with the data on the list of pages requested, and if the received page is a requested page, then the filter 12 sends the memory 14 a storage control signal. The memory 14 stores the received page given by the decoder 11 when it receives the storage control signal. Upon the user""s request, a page stored in the data memory 14 is then stored in the display memory 15. The display means 16 constantly reads the contents of the display memory 15 and copies them on the screen 17.
When the user requests the display of a page on the screen, the mean time needed for the appearance of this page on the screen depends essentially on the size of the data memory 14. If a requested page is already stored in the data memory 14, then the display of this page is immediate. If, on the contrary, the requested page is not stored in the memory 14, then it is necessary to wait for reception of this page to see it appear on the screen. The wait may be relatively lengthy, given that the same page is broadcast approximately every 40 seconds.
The size of the memory 14 is highly variable, between 1 and 500 kilobytes, for example. For an average quality circuit 10, a memory of about 10 kilobytes is used, which enables the storage of about ten service pages. This choice results from a compromise between, firstly, the size (in terms of silicon surface area) and the cost of the memory, and secondly, the user""s comfort which is directly related to the mean time of display of a page. The choice of the elements of the list of the requested pages reduces the mean display time of a page, given that there is a very high probability that the user will request the display of one of these pages.
One problem appears, however, when the data memory 14 is too small to store all the pages of the list of the requested pages. All the pages of the list are stored in the memory as soon as they are received. Consequently, if a new requested page is received and if the space that is still free in the memory 14 is not enough to store the new received page, then either a previously stored page is totally or partially erased and replaced by the last page received, or else the last page received is not stored.
This is particularly inconvenient for the user because it is possible that the pages closest to the page being displayed will be erased during the storage of a new requested page. When the user requests the display of a nearby page of the displayed page (the most probable assumption), and if this page to be displayed has been totally or partially erased from the memory 14, or if this page to be displayed has not been stored, then the user must wait for the next transmission of this page to be displayed before seeing it appear on the screen. The user""s comfort is thus seriously degraded.
It may be recalled that the expression xe2x80x9ccloser toxe2x80x9d must be understood as follows: a page 1 is closer to a page X then a page Y2 if, when the page X is displayed, the probability that the user will request the display of the page Y1 is higher than the probability that the user will requests the display of the page Y2. For example, pages 199, 201 are considered to be closer to page 200 than pages 198, 202.
This problem of the erasure of data in the memory arises frequently if the list of the requested pages contains a variable number of references. This problem is also frequent with the new versions of teletext services for which the size (in terms of memory space) of each page may vary greatly. It is then very difficult to foresee the exact number of pages that the memory 14 may contain, because this number can vary as a function of the pages to be stored.
To resolve this problem, it is possible to limit the list of the pages requested to a fixed number of pages on the basis, for example, of a page size equal to the maximum size of a page of a service. In this case, the total number of stored pages will be necessarily small. This increases waiting risks for the viewer, and therefore, reduces comfort.
It is also possible to choose a mean number of requested pages. In this case, the risks of the overwriting of data in the memory are high, and therefore, the waiting risks for the viewer are also increased. Thus, the approaches currently used for the storage of teletext pages are not satisfactory because they cannot be used for the most efficient optimizing of the contents of the memory while at the same time achieving the maximum reduction of possible discomforts for the viewer.
In view of the foregoing background, an object of the present invention is to reduce the average display time for a teletext page to heighten a user""s comfort, and to achieve this without increasing the size of the data memory as compared with existing circuits, namely by limiting their size (in terms of silicon surface area) and the cost of the circuit.
Another object of the invention is to implement a storage method and make an associated storage circuit that takes account of the size of the pages to be stored and the probability that a page will be requested by the viewer.
Another object of the invention is to implement a method of storage and make an associated storage circuit that keeps in memory at least the pages closest to the page under displayed, and does so regardless of the page being displayed.
These and other objects, advantages and features of the invention are provided by a method for the storage of pages of a teletext service, with at least one page being received by a storage circuit of a television receiver, and the storage circuit comprises a data memory to store the at least one received page. The method comprises a step for the extraction of a reference number from the received page, a step to check whether the received page is a requested page, and a step for the storage of the received page.
According to the invention, the method also comprises a step for evaluating the contents of the data memory to decide whether the received page must be stored as a function of the free space in the data memory and the importance of the received page. The evaluation step is performed after the checking step, and the storage step is carried out if the received page has to be stored.
Thus, the method of the invention takes account of the place available in the data memory, and the importance of the received page to decide whether the received page must be stored or not. A page X is said to be important if it is close to a selected page Y (for example, a displayed page) that is selected by a user, namely if the probability is high that the user will select the page Y after having selected the page X.
Thus, with the invention, the important pages are stored as a priority if the space in the data memory is limited. Consequently, the probability that the user will ask for a page not contained in the data memory is reduced and the user""s comfort is increased accordingly.
According to one embodiment, the step for evaluating the contents of the memory comprises the following steps. Step E31 is the comparison of the size of the free space in the data memory with the size of the received page and then, if the size of the free space is smaller than the size of the received page, then step E32 is performed, which is the study of the importance of the received page.
The importance of the received page is taken into account only if the free space in the memory is not enough to store the received page. The step E32 for studying the importance of the received page can be sub-divided as follows. Step E32.1 is the comparison of a priority number of the received page with a priority variable equal to the priority number of the lowest-priority page contained in the data memory. If the priority number of the received page is smaller than the priority variable, then a new extraction step is performed, otherwise, the step E32 ends.
As the case may be, the step E3 for evaluating the contents of the memory also comprises a step E33 for the erasure of the lowest-priority page contained in the data memory. The erasure step E33 is performed after the step E32, and the erasure step E33 is performed if the received page has a priority number that is more important than the priority variable. The step E3 is complemented by an updating step E34 for updating the priority variable performed after the step E31, or after the step E32, or after the step E33.
According to another embodiment, a new step E31 is performed after the step E34 if a step E32 has been performed during the step E3. This makes it possible to verify that the released memory space is sufficient to store the received page.
The invention also relates to a television receiver comprising storage means to implement the data storage method described above.
The invention also relates to a circuit for the reception and display of teletext pages, with the circuit comprising a storage circuit comprising a decoder to receive a page of a teletext service and to extract therefrom a reference number of the received page, and a data memory to store the received page.
According to the invention, the storage circuit also comprises a filter to verify whether the received page is a requested page, and to provide the priority number of the received page. A memory management circuit receives the priority number of the received page, and causes the received page to be stored if the size of a free space of the data memory is sufficient, or if not, then the study of the importance of the received page is performed.
Preferably, the memory management circuit comprises means to compare the size of the free space of the data memory with the size of the received page. To study the importance of the received page, the memory management circuit preferably has means to compare the priority number of the received page with a priority variable equal to a priority number of a lower-priority page contained in the data memory.
Finally, the invention also relates to a television receiver comprising a circuit for the reception and display of teletext pages as described above.